Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. In a so-called “color flow” mode, the flow of blood or movement of tissue can be imaged. Conventional ultrasound flow imaging techniques use either the Doppler principle or a time-domain cross-correlation method to estimate the average flow velocity, which is then displayed in color overlaid on a B-mode image.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of back-scattered ultrasound waves may be used to measure the velocity of the back-scattered waves from tissue or blood. The change or shift in back-scattered frequency increase when blood flows towards the transducer and decreases when blood flows away from the transducer. The Doppler shift may be processed to estimate the average flow velocity, which is displayed using different colors to represent speed and direction of flow. The color flow velocity mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent each sample volume's velocity. Hence such Doppler shifts measured in a color flow imaging mode may be referred to hereinafter as color flow data.
The sensitivity of an ultrasound imaging system to color flow data, or the ability of the ultrasound imaging system to detect Doppler shifts from ultrasonic waves reflected from flowing blood, depends on a color flow steering angle, where the color flow steering angle comprises an angle between a color flow region of interest and a B-mode image. For example, a color flow steering angle of zero corresponds to a color flow region of interest completely overlapping the underlying B-mode image, and results in low color flow sensitivity. As the color flow steering angle increases, the color flow sensitivity increases. However, if the color flow steering angle is too large, the color flow region of interest will not overlap the B-mode image at all and the color flow sensitivity is moot.
Furthermore, a user of an ultrasound imaging system may adjust the focal depth of the ultrasound image in order to view particular structures within a patient. For shallow focal depths, a large color flow steering angle will result in high color flow sensitivity. As the focal depth is increased, however, the color flow steering angle must be decreased to ensure that the color flow region of interest overlaps the underlying B-mode image.
Thus, for conventional ultrasound imaging systems, a user needs to perform multiple operations—adjusting a focal depth and subsequently adjusting a color flow steering angle—to obtain optimized images, which is time consuming and user dependent. Furthermore, an inexperienced user may generate suboptimal images, thereby increasing risk of an incorrect diagnosis. Further still, some ultrasound imaging systems may be portable, handheld devices and therefore may include a limited number of user control inputs. As a result, the process of calibrating a color flow imaging mode may be more complex, further increasing the difficulty of obtaining optimized ultrasound images and increasing the risk of an incorrect diagnosis. The inventors have recognized the above issues and have devised several approaches to address them.